Offshore Wind Turbine Monopile Foundations Research Articles

Development and prediction of scour around offshore wind turbine monopile foundations in ebb and flow tides

Monopile foundations are commonly used to support offshore wind turbines (OWTs) in nearshore shallow waters, where the periodic ebb and flow of tides can cause severe seabed erosion around these foundations. Accurately predicting the progression of scour depth under these tidal conditions is crucial for the safety of OWTs. While a few empirical formulas for equilibrium scour depth in periodic tides have been proposed, they often overlook the backfilling effects during their derivation. Consequently, these methods fail to predict the progression of scour depth characterized by recurring scouring and backfilling processes. In this study, the scour progression under different tidal patterns, including square tides, symmetrical sinusoidal tides, and asymmetrical sinusoidal tides with various periods, is investigated using numerical methods. It is indicated by the results that deeper scour depths are produced by symmetrical tides compared to asymmetrical tides, while the highest scour depth is observed under square tides, potentially slightly exceeding that under unidirectional currents. Additionally, asymmetric scour depths at the front and back sides of the pile can result from the asymmetry of tides. Although scour progression is affected by tide periods, their impact on the equilibrium scour depth is limited. Furthermore, a new concept called normalized cumulative effective flow intensity is introduced, which plays a primary role in governing equilibrium scour depth. Based on this concept, prediction methods for both equilibrium scour depth and scour progression under sinusoidal tides, which closely resemble natural tidal patterns, are proposed. The accuracy of these methods is validated by experimental data from previous studies. These findings not only provide a method to optimize the design of monopile burial depth but also aid in the development of appropriate maintenance measures to prevent scour-induced damage to OWT foundations over their long lifespans.

Numerical Investigation of Local Scour Protection around the Foundation of an Offshore Wind Turbine

The pile foundations of offshore wind turbines face serious problems from scour damage. This study takes offshore wind turbine monopile foundations as the research object and proposes an innovative anti-scour device for the protection net. A numerical simulation research method based on CFD-DEM was used to model the local scour of the pile foundation and protection net. The validity of the numerical model was verified by comparing the simulation results of the local scour of the pile foundation under the condition of clear water scour and the results of the flume test. The permeability rate was defined to characterize the overwatering of the protection net, and numerical simulations were performed for protection nets with permeability in the range of 0.681 to 0.802. The flow field perturbations, changes in washout pit morphology, and changes in washout depth development due to the protective netting were also analyzed. It was found that the protection net can effectively reduce the flow velocity around the pile, cut down the intensity of the submerged water in front of the pile, and provide scouring protection. Finally, the analysis and summary of the protection efficiency of the different protection nets revealed that the protection efficiency within the nets was consistently the highest. On the outside of the net, the protection efficiency is poor at a small permeability rate but increases with an increasing permeability rate.

Performance of riprap armour at vibrating offshore wind turbine monopile foundations

Performance of riprap armour at vibrating offshore wind turbine monopile foundations

Statistical modeling of fully nonlinear hydrodynamic loads on offshore wind turbine monopile foundations using wave episodes and targeted CFD simulations through active sampling

AbstractAccurately determining hydrodynamic force statistics is crucial for designing offshore engineering structures, including offshore wind turbine foundations, due to the significant impact of nonlinear wave–structure interactions. However, obtaining precise load statistics often involves computationally intensive simulations. Furthermore, the estimation of statistics using current practices is subject to ongoing discussion due to the inherent uncertainty involved. To address these challenges, we present a novel machine learning framework that leverages data‐driven surrogate modeling to predict hydrodynamic loads on monopile foundations while reducing reliance on costly simulations and facilitate the load statistics reconstruction. The primary advantage of our approach is the significant reduction in evaluation time compared to traditional modeling methods. The novelty of our framework lies in its efficient construction of the surrogate model, utilizing the Gaussian process regression machine learning technique and a Bayesian active learning method to sequentially sample wave episodes that contribute to accurate predictions of extreme hydrodynamic forces. Additionally, a spectrum transfer technique combines computational fluid dynamics (CFD) results from both quiescent and extreme waves, further reducing data requirements. This study focuses on reducing the dimensionality of stochastic irregular wave episodes and their associated hydrodynamic force time series. Although the dimensionality reduction is linear, Gaussian process regression successfully captures high‐order correlations. Furthermore, our framework incorporates built‐in uncertainty quantification capabilities, facilitating efficient parameter sampling using traditional CFD tools. This paper provides comprehensive implementation details and demonstrates the effectiveness of our approach in delivering reliable statistics for hydrodynamic loads while overcoming the computational cost constraints associated with classical modeling methods.

Numerical study on vibratory extraction of offshore wind turbine monopile foundations under sandy seabed condition

The wind industry has experienced a rapid growth in Europe over the last decades. The early generation turbines were designed for a life of 20–25 years. Decommissioning of offshore wind turbines is becoming more important since many of these installed assets are approaching their end of lifetime. In this study, using vibratory extraction of monopile foundations instead of current practice of cutting them is investigated numerically. Correct estimation of extraction force helps operators to choose suitable vibro-hammer and vessel (or crane-barge), which leads to reduction of decommissioning costs. A Coupled Eulerian-Lagrangian (CEL) approach of ABAQUS/Explicit combined with a modified Mohr-Coulomb (MMC) model are used to find the pile shaft resistance during total removal operation under saturated dense sand condition. The MMC model captures the nonlinear pre-peak hardening and post-peak softening of the dense sand which is not modelled by conventional Mohr-Coulomb model. The VUSDFLD subroutine, which is a user-defined framework, has been used to implement the MMC model into CEL analysis. A parametric study is conducted to analyze how the characteristics of the vibro-hammer, such as its frequency, eccentric moment, and the extraction rate influence the results. The present numerical results show that using proper frequency results in reduction of soil resistance to less than 25% of the initial resistance. However, appropriate hammer with enough eccentric moment and suitable extraction rate, are vital to ensure soil degradation. The results show that the proposed methodology is both robust and straightforward and it has the potential to reduce computational time which is efficient for engineering applications.

Experimental study of collar protection for local scour reduction around offshore wind turbine monopile foundations

Experimental study of collar protection for local scour reduction around offshore wind turbine monopile foundations

Cyclic overlay model of p–y curves for laterally loaded monopiles in cohesionless soil

Abstract. The bearing behaviour of large-diameter monopile foundations for offshore wind turbines under lateral cyclic loads in cohesionless soil is an issue of ongoing research. In practice, mostly the p–y approach is applied in the design of monopiles. Recently, modifications of the original p–y approach for monotonic loading stated in the API regulations have been proposed to account for the special bearing behaviour of large-diameter piles with small length-to-diameter ratios. However, cyclic loading for horizontally loaded piles predominates the serviceability of the offshore wind converters, and the actual number of load cycles cannot be considered by the cyclic p–y approach of the API regulations. This research therefore focuses on the effects of cyclic loading on the p–y curves along the pile shaft and aims to develop a cyclic overlay model to determine the cyclic p–y curves valid for a lateral load with a given number of load cycles. A stiffness degradation method (SDM) is applied in a three-dimensional finite element model to determine the effect of the cyclic loading by degrading the secant soil stiffness according to the magnitude of cyclic loading and number of load cycles based on the results of cyclic triaxial tests. Thereby, the numerical simulation results are used to develop a cyclic overlay model, i.e. an analytical approach to adapt the monotonic (or static) p–y curve to the number of load cycles. The new model is applied to a reference system and compared to the API approach for cyclic loads.

Countermeasures for local scour at offshore wind turbine monopile foundations: A review

Local scour at monopile foundations of offshore wind turbines is one of the most critical structural stability issues. This article reviews the contemporary methods of scour countermeasures at monopile foundations. These methods include armouring countermeasures (e.g., riprap protection) to enhance the anti-scour ability of the bed materials and flow-altering countermeasures (e.g., collars and sacrificial piles) to reduce downflow or change flow patterns around the monopiles. Stability number and size-selection equations for riprap armour layers are summarised and compared. Moreover, other alternative methods to riprap are briefly introduced and presented. A typical graph of the scour depth reduction with different collar sizes and elevations under specific test conditions is summarised and compared with a plot for a pile founded on a caisson. Reduction rates for different flow-altering countermeasures, including the collar, are listed and compared. A newly developed soil improvement method, namely microbially induced calcite precipitation (MICP), is also reviewed and introduced as a scour protection method. As a popular bio-soil treatment method, MICP has a good potential as a scour countermeasure method. Bio-soil treatment methods and traditional armouring methods are defined as active and passive soil enhancement scour countermeasures, respectively.

Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils

Most of the reported centrifuge tests available in the existing literature on offshore wind turbine foundations are focused on the behaviour of monopiles in sands, but very few studies on clayey soils can be found, due to the very long saturation and consolidation periods required to properly conduct experiments in such materials. Moreover, most of the reported numerical simulations using finite element analyses have been validated with monotonic centrifuge tests only. In this research, both monotonic and cyclic performance of offshore wind turbines in clay are validated and justified. The relationship between the monopile rotation in clays and the geometry and strength of the soil has been found and quantified. A prediction of the rotation for a high number of cycles of loading, based on the one experienced by the pile during the first cycle, can be obtained using the correlation derived in the paper. For those cases in which the rotation does not reach a steady value after a high number of cycles, the cumulative rate has been found significantly larger than the prediction conducted with standard analytical methods. A new design methodology for the design of offshore monopile foundations in clay is presented.

Material pre-straining effects on fracture toughness variation in offshore wind turbine foundations

S355 structural steel is a commonly used material in the fabrication of foundation structures of offshore wind turbines, which are predominantly supported using monopiles. During the manufacturing process of monopile foundations, S355 steel plates are pre-strained via a three point bending and rolling process, which subsequently changes the mechanical, fatigue and fracture properties of the material. The aim of this study is to investigate the variation in fracture toughness of S355 material by considering a range of pre-strain levels induced during the manufacturing process. Fracture toughness tests have been performed on compact tension specimens made of the as-received, 5% and 10% pre-strained S355 material. The test results have shown that the fracture toughness of the material decreases as the percentage of pre-straining increases. An empirical correlation has been derived between the yield strength of the material, the plastic pre-strain level and the fracture toughness values. The drawn relationship can potentially be utilised in the life assessment of offshore wind turbine monopile foundations to give a relatively accurate estimate of the remaining life by considering realistic values of fracture toughness post-fabrication, which results in better informed design and assessment.

On the performance of monopile weldments under service loading conditions and fatigue damage prediction

AbstractThick weldments used in offshore structures frequently act as fatigue crack initiation sites due to stress concentration at weld toe as well as weld residual stress fields. This paper investigates the cyclic deformation behavior of S355 G10+M steel, which is predominantly used in offshore wind applications. Owing to the vast size difference of monopile structure and weld cross‐section, a global–local finite element (FE) method was used, and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using supervisory control and data acquisition (SCADA) and wave buoy techniques were used in the FE model. A nonlinear isotropic–kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal (BM) as well as cross‐weld specimen tests. The tests revealed that the S355 G10+M BM and weld metal (WM) undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the local stress at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.

Offshore wind turbine monopile foundations: Design perspectives

Perspectives on design of offshore wind turbine (OWT) monopile foundations are provided in this paper by logically adopting optimal analysis choices with a balance between accuracy and computational efficiency. A newly developed analysis framework is used for this purpose. The analysis framework consists of dynamic analysis with linear viscoelastic soil and static analysis with nonlinear elastic soil. The framework is used to show that the Timoshenko beam theory produces most accurate monopile response although the Euler-Bernoulli beam theory produces optimal results maintaining a balance between accuracy and computational efficiency. The rigid beam theory can be used only under very restricted soil and monopile conditions. The analysis further demonstrates that static analysis is sufficient and optimal in producing monopile-soil stiffness values that can be used for determining the natural frequency of vibration of the OWTs. Moreover, nonlinear elastic analysis is sufficiently accurate and quick to produce monopile responses that can be used in design and there is no need for elasto-plastic analysis. The aspect of cyclic degradation of soil stiffness over the design life of monopiles is not considered. Thus, an optimal pathway for monopile design is described using the newly developed analysis framework and demonstrated through a design example.

Validation and Application of a New Software Tool Implementing the PISA Design Methodology

The PISA (Pile Soil Analysis) research project has resulted in a new methodology for the design of offshore wind turbine monopile foundations. A new software tool called PLAXIS Monopile Designer (MoDeTo) has been developed that automates the PISA design methodology. It facilitates the calibration of the so-called soil reaction curves by automated three-dimensional finite element calculations and it allows for a quick design of monopiles using the calibrated soil reaction curves in a one-dimensional finite element model based on Timoshenko beam theory. The monopile design approach has been validated for sand- and clay-type soils which are common in North Sea soil deposits. The paper presents a validation exercise based on the PISA research project proposal of a rule-based parametric model—General Dunkirk Sand Model (GDSM)—for Dunkirk sand as well as an application of the tool for a project involving an offshore wind turbine on a monopile foundation in sandy layered soil in which the PISA design is compared to the conventional API design. The paper concludes with a discussion of the results and the differences between the various methods.

Flume Tank Testing of Offshore Wind Turbine Dynamics with Foundation Scour and Scour Protection

Scour erosion processes can occur at seabed level around offshore wind turbine monopile foundations. These scour processes are often especially severe at sites where mobile sediments, such as sands, are present in the superficial seabed soils. Loss of local soil support to the monopile, caused by scour erosion, can lead to significant changes in the dynamic characteristics of the wind turbine support structure. This can result in accelerated fatigue damage, owing to the applied cyclic loads from the wind turbine generator, especially at the rotor frequency. Although scour erosion can be controlled by appropriate scour protection systems, there is a lack of knowledge to support the design and optimization of these protection measures, to ensure that the dynamic performance of the wind turbine support structure remains within acceptable limits. This paper describes an experimental campaign conducted on a 1:20 scale model of a driven monopile foundation and wind turbine support structure, founded in a prepared sand test bed in the Fast Flow Facility flume (HR Wallingford, UK). Scour processes were induced by applying cycles of flow. Experiments were conducted to investigate the influence that these scour processes, and selected concepts for preventative and remedial scour protection, have on the dynamic characteristics of the monopile–tower system. The paper describes the experimental procedures that were adopted, and provides an assessment of the results.

A Feasibility Study for Using Fishnet to Protect Offshore Wind Turbine Monopile Foundations from Damage by Scouring

Offshore wind turbine monopile foundations are subjected to complex wind, wave, and flow coupling effects, which result in seabed scouring around the monopile. The consequent scour pits threaten the reliability, safety, and load-carrying capacity of the monopile. In order to develop a cost-effective measure to mitigate such an issue, a new countermeasure device, named “fishnet”, is studied in this paper using a combined approach of numerical simulations and experimental tests. In the research, the size of the fishnet, diameter of the fishnet thread, and the installation height of the fishnet were optimized in order to achieve the best protection to the monopile foundation. In the paper, both numerical simulations and laboratory tests proved the effectiveness of the proposed “fishnet” in reducing the scour around the wind turbine monopile foundations. Moreover, its contribution to erosion reduction can be further enhanced via optimization. It was found that, after optimization, the maximum shear force on the seabed could be reduced by 14% in the numerical study, and the maximum depth of the scour pit could be reduced by 38.2% in laboratory tests.

New shape function solutions for fracture mechanics analysis of offshore wind turbine monopile foundations

Offshore wind turbines are considered one of the most promising solutions to provide sustainable energy. The dominant majority of all installed offshore wind turbines are fixed to the seabed using monopile foundations. To predict the lifetime of these structures, reliable values for shape function and stress intensity factor are needed. In this study, a new equation is developed through finite element simulations which have been performed for a wide range of monopile geometries with different dimensions, crack lengths and depths, to evaluate shape function and stress intensity factor solutions for monopiles. The new solutions have been verified through comparison with the existing solutions provided by Newman & Raju for small hollow cylinders. The empirical shape function solutions developed in this study are employed in a case study and the results have been compared with the existing shape function solutions. It is found that the old solutions provide inaccurate estimations of fatigue crack growth in monopiles and they underestimate or overestimate the fatigue life depending on the shape function solution employed in the structural integrity assessment. The use of the new solution will result in more accurate monopile designs as well as life predictions of existing monopile structures.

Monopiles subjected to uni- and multi-lateral cyclic loading

Offshore wind turbines are subjected to significant environmental loads from a combination of current, wind and wave action. Under such conditions, the directions of these environmental loads vary over the service life of the structure and therefore the cyclic lateral loading on the foundation also changes direction. The work reported in this paper examines the effects of multi-directional loading on the performance of offshore wind turbine monopile foundations. Tests were carried out in a model sand bed and a mobile loading platform was manufactured to apply loading on the pile in various directions. Tests were also carried out where the cyclic loading was applied under both one-way and two-way loading. The observations indicate significant differences in the stiffness of monopiles between uni-directional and multi-directional lateral cyclic loading. Multi-directional lateral cyclic loading generally results in higher displacements and lower stiffness compared to uni-directional loading, most likely due to shear deformation of a larger volume of soil mass adjacent to the pile.

Ship collision analysis on offshore wind turbine monopile foundations

An offshore wind farm can be located close to traffic lanes of commercial and passenger ships, which may lead to possible collision. The work presented in this paper aims to understand both the crushing behaviour and the nacelle dynamics of a monopile offshore wind turbine when impacted by a ship. Another objective was to deeply investigate the influence of various parameters like ship impact velocity and location, wind direction, soil stiffness and deformability of the striking ship.First, nonlinear numerical simulations of ship - Offshore Wind Turbine (OWT) collisions have been carried out with a rigid striking ship for a better understanding of the OWT's structural behaviour during collision. Different configurations for the wind turbine's structure have been used in order to highlight the modifications in behaviour induced by changing the soil conditions or the loading scenario. The resulting resistant force and internal energies have been compared as well as the tower top (nacelle) displacements and accelerations. Then, another series of simulations have been performed with a deformable ship in order to investigate the influence of the deformability of the striking ship on the OWT's behaviour.It was shown that a slight variation of the impact velocity can lead to consequences ranging from minor damage of the OWT to collapse. Furthermore, the behaviour of the OWT during collision is highly sensitive to wind loads as it was shown that in some cases the OWT can collapse for an impact velocity of only 3 m/s and, in the worst case, can fall directly on the ship. The results obtained showed that it is important to account for the soil flexibility when performing collision simulations because considering the monopile as being clamped at the base would lead to an overestimation of the plastic deformation of the collided structure. Moreover, when the OWT is collided by a deformable striking ship, the deformations of the OWT are 2 times smaller comparing to the deformations caused by impact with a rigid ship and the structure can withstand collisions without collapsing with an impact velocity up to 6 m/s.

P-y-ẏ curves for dynamic analysis of offshore wind turbine monopile foundations

The well-known p-y curve method provides soil-structure interaction that is independent of the load rate. In this paper an improved p-y curve method is proposed by considering the influence of the excitation frequency. For this purpose, a two-dimensional finite-element program is developed for analysis of a segment of an offshore monopile foundation placed in different depths. The intended use of the model is analyzes of offshore wind turbines in operation where small-magnitude cyclic response is observed in addition to the quasi-static response from the mean wind force. The response to small-magnitude cyclic loading is analyzed by employing coupled equations based on the u-P formulation, i.e. accounting for soil deformation as well as pore pressure. Thus, the paper has focus on the effects of drained versus undrained behavior of the soil and the impact of this behavior on the stiffness and damping related to soil-structure interaction at different load frequencies. In order to enable a parameter study with variation of the soil properties, the constitutive model is purposely kept simple. Hence, a linear poroelastic material model with few material parameters is utilized. Based on the two-dimensional model, linear p-y-ẏ curves are extracted for the lateral loading of monopiles subjected to cyclic loads. The developed code is verified with findings in the literature.

Recovery-based error estimation in the dynamic analysis of offshore wind turbine monopile foundations

Offshore wind turbine foundations are affected by cyclic loads due to oscillatory kinematic loads, such as those from wind, waves, and earthquakes. Monopiles are often used as a foundation concept for offshore windmill turbines. In this study, coupled dynamic equations with the u-P formulation for low-frequency load are considered for an offshore wind turbine monopile foundation, to present the response in terms of pore water pressure (PWP), stress and strain distribution in an elastic porous medium at regions around the monopile foundation. Different stress recovery techniques based on the Zienkeiwicz–Zhu (ZZ) error estimator namely, super-convergent patch recovery (SPR), weighted super-convergent patch recovery (WSPR), and L2-projection techniques are also investigated to recover the stresses at nodal points in the finite element method. To estimate errors in the time domain when performing transient simulations, three recovery processes are used with different meshes. The convergence of the dynamic problem is also studied. The results are verified with findings in the literature, revealing that the time period of effective stresses follows the applied load frequency. In conclusion, the history of the shear stress can have an important effect on the shear stress distribution, making it asymmetric in the time domain.